Blocking composition for use in subterranean formation

ABSTRACT

A polymer composition for pumping downhole to gel in a subterranean formation comprises a water-soluble copolymer of (i) at least one non-acidic ethylenically unsaturated polar monomer and (ii) at least one copolymerisable ethylenically unsaturated ester; and an organic gelling agent therefor.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to polymeric compositions and their usefor blocking subterranean formations in oil and gas fields.

2. Discussion of Realted Art

Oil and gas fields produce water as well as oil and/or gas, especiallywhen the well is depleted. In addition. secondary recovery techniquessuch as water flooding to stimulate production of oil involve injectionof water under pressure at a distance from a production well to squeezethe oil out. However, in both cases the water moves in the formationalong least hindered paths, so that the recovery technique may beinefficient, and in the direct recovery increased proportions of waterare produced.

To enhance reservoir conformance control, i.e. to mobilise the oil thatmay be present in less permeable areas, blocking agents may be injectedto obstruct the high permeability channels thereby encouragingpreference for liquid movement via the lower permeability channels.Among known blocking agents are polymer gels, in particular gels ofpolyacrylic acid or polyacrylamide/polyacrylic copolymers, cross-linkedwith chromium ions as disclosed, for example, in U.S. Pat. Nos.4,744,418 and 4,844,168. The copolymer, mixed with cross-linker, isinjected into the formation from the production well, often after aflood of cold water to pre-cool the formation rock, to stop prematurecross-linking and gelling before the mixture reaches its desiredposition. Much work has been described to reduce the rate ofcross-linking, by reducing the activity of the cross-linking metal ion,e.g. by co-ordinating the chromium with a ligand, e.g. lactate. Othercross-linking agents which have been used are metals such as zirconiumand other transition metals.

Whilst the use of metal cross-linking agents is reasonably satisfactoryin practice, there are certain disadvantages. For example, there can beenvironmental effects with the use of metal ions, particularly withchromium. For some rocks, such as carbonate rocks, the metals used ascross-linking agents are absorbed by the rocks and are hydraulicallyunstable at temperatures above 70° C. and so cannot function properly,thus reducing the effectiveness of the blocking polymers.

U.S. Pat. No. 4,773,481 describes a process for reducing thepermeability of a subterranean formation by the gelation ofwater-soluble polymers of polyalkyleneimines and polyalkylenepolyamineswith certain polymeis which are anionic or hydrolysable to form anionicpolymers and which are cross-linked by the water-soluble polymers.Examples of the anionic or non-ionic polymers are polyacrylamide andalkylpolvacrylamides, copolymers of polyacrylamide andalkylpolyacrylamides with ethylene, propylene and styrene, polymaleicanhydride and polymethacrylate and hydrolysis products thereof. Asdescribed in the patent, when the water-soluble polymer and the anionicpolymer are mixed, a viscous gel is quickly formed. In use, a solutionof the water-soluble polymer is pumped into the subterranean formationfirst, followed by water to cleanse the bore of the water-solublepolymer to prevent premature gelling upon introduction of the anionicpolymer. and finally the anionic polymer is introduced.

This three step procedure has a number of disadvantages in practice butit is necessary because the water-soluble polyalkyleneimine orpolyalkylenepolyamine reacts very quickly with the anionic polymer andhence cannot be premixed before injection without substantial risk ofpremature gelation. To some extent, this problem can be reduced by usinga non-ionic polymer which is hydrolysable to an anionic polymer at arelatively slow rate, but this limits the choice of materials and is notgenerally satisfactory. So far as we are aware, the process of U.S. Pat.No. 4,773,481 has either not been commercially utilised, or has onlybeen used to a small extent, presumably because of the substantialpractical difficulties involved.

SUMMARY OF THE INVENTION

We have now found a way of avoiding the use of metal ion cross-linkingagents and of controlling the gelling rate of polymers whereby premixesof polymer and gelling agent can be made and safely injected into adownhole formation without serious risk of premature gelation.

In one aspect, the invention provides a composition for use in asubterranean formation, which composition comprises a water-solublecopolymer comprising

(i) at least one non-acidic ethylenically unsaturated polar monomer and

(ii) at least one copolymerisable ethylenically unsaturated ester;

and at least one organic gelling agent, characterized in that theorganic gelling agent is a polyalkyleneimine, polyfunctional aliphaticamine, an aralkylamine or a heteroaralkylamine.

The organic gelling agents are free from metal ions, and are preferablywater-soluble polymers capable of cross-linking the copolymers. Amongthe preferred water-soluble polymers for use as gelling agents arepolyalkyleneimines, polyalkylenepolyamines and mixtures thereof.Additional details concerning these polymers and their preparation aredisclosed in U.S. Pat. No. 3,491,049. The preferredpolyalkylenepolyamines are the polymeric condensates of lower molecularweight polyalkvlenepolyamines and a vicinal dihaloalkane. Thepolyalkyleneimines are best illustrated by polymerized ethyleneimines orpropyleneimine. The polyalkylenepolyamines are exemplified bypolyethylene and polypropylenepolyamines.

Other gelling agents which can be used include water-solublepolyfunctional aliphatic amines, aralkylamines and heteroaralkylaminesoptionally containing other hetero atoms.

In a second aspect. the invention provides a method of conformancecontrol of a subterranean reservoir, which comprises:

(a) injecting into a formation an aqueous solution of a composition ofthe invention;

(b) allowing the solution to flow through at least one permeable zone insaid formation; and

(c) allowing the composition to gel.

In the method of the invention. it is generally unnecessary to have anypre-cool step, especially in wells with bottom hole temperatures up toabout 120° C. As the solution is pumped downhole and permeates into thezone, it heats up and eventually reaches the downhole temperature afterwhich gelling occurs. The permeability of the zone is preferably highbut it can be down to about 60 mD.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a graphical plot of viscosity versus time for a composition ofthe present invention.

FIG. 2 is a graphical plot of viscosity versus time for a composition ofthe present invention.

FIG. 3 is a graphical plot of viscosity versus time for comparisonutilizing known compositions.

FIG. 4 is a graphical plot of viscosity versus time for comparisonutilizing known compositions.

DETAILED DESCRIPTION OF THE INVENTION

In the composition of the invention, the copolymer is formed from atleast one polar monomer, preferably from 1 to 3 monomers, and at leastone, preferably from 1 to 3, esters, and comprises structural unitsderived from said monomer(s) and ester(s). Most preferably, thecopolymer consists essentially of said structural units. The ester ispreferably substantially neutral as a fully esterified derivative of anacid, i.e. complete ester, rather than a partial ester with free acidgroups.

The presence of the ester moiety in copolymers of the invention is vitalsince it delays the gelling reaction and thus enables the copolymer tobe premixed with gelling agent before being pumped downhole. We preferthat the ester group be such as to provide steric hindrance and, forthis purpose, bulky ester groups such as t-butyl, for example, arepreferred. The precise delay in cross-linking and gelation caused by theester group will vary from copolymer to copolymer, as will be clear tothose skilled in the art. Some experimental trial may, therefore, benecessary to determine the optimum with any particular copolymer. Thenature and amount of the ester will be such as to provide a delay in thegelation (compared to a homopolymer omitting any ester component),sufficient for example to enable a premix to be pumped into a formationwithout premature gelling.

The ethylenically unsaturated esters used in the copolymers are usuallyformed from a hvdroxyl compound and an ethylenically unsaturatedcarboxylic acid. The ethylenically unsaturated group is preferably inthe alpha-beta or beta-gamma position relative to the carboxyl group,but it may be further distant. Preferred acids have 3-20 carbon atomse.g. 3 to 12 and include. for example. alkenoic and aralkenoic acidswith 3 to 6 or 9 to 12 carbon atoms, respectively. Examples of theseacids are acrylic, methacrylic. crotonic and cinnamic acids. Thehydroxyl compound is usually an alcohol and may be of formula ROH. whereR is a hydrocarbyl group. Preferred hydrocarbyl groups are alkyl groupsof 1 to 30 or 2 to 30 such as 1 to 6, 2 to 6, 7 to 30 or 7 to 24 carbonatoms; alkenyl groups of 2 to 20 carbon atoms such as 2 to 6 carbonatoms; cycloalkyl groups of 5 to 8 carbons; aryl groups such as aromatichydrocarbyl groups having 6 to 20 carbon atoms. and aralkyl groups of 7to 24 carbon atoms. Specific examples of R groups are methyl, ethyl,propyl, butyl, amyl, hexyl, octyl, 2-ethylhexyl and decyl (including allstereoisomers), allyl, cyclohexyl, palmityl, stearyl, phenyl and benzyl.The R group may also be a hydrocarbyl group substituted by at least one,e.g. 1 to 3 substituents, especially hydroxyl, ether, and thio ethergroups. Electron donating group substituents are preferred. Ethersubstituents are preferred, especially alkoxy, aryloxy and aralkoxy, inwhich the alkyl, aryl and aralkyl groups may be as described above.Preferably, the substituent is on the same carbon atom of the R group asis bonded to the hydroxyl group in the hydroxyl compound; alkoxymethyland aralkoxy methyl groups are preferred. The hydroxyl compound may be aprimary, secondary, iso or tertiary compound, especially with a tertiarycarbon atom bonded to the hydroxyl group, e.g. tert-butyl and trityl.The group R may also comprise a heterocyclic group either for bondingdirectly to the hydroxyl group of ROH or separated therefrom by analkylene group, e.g. of 1 to 4 carbon atoms such a methylene. Thus,group R may be a saturated or unsaturated heterocyclic or heterocyclicalkylene group. e.g. of 3 to 8 carbon atoms and at least one, e.g. oneor two ring heteroatoms selected from O, N and S, especially O and/or N.Examples of such groups are furyl, tetrahydrofuryl, furfuryl andtetrahydrofurfuryl, pyranyl and tetrahydropyranyl. Most preferred Rgroups are tert-butyl, trityl, methoxymethyl, benzyloxymethyl andtetrahydropyranyl; stearyl, isopropyl, ethyl and methyl may also bepreferred.

The preferred ester is t-butyl ester.

The ethylenically unsaturated ester may also be derived from a hydroxylcompound. e.g. of formula ROH, and an ethylenically unsaturatedsulphonic or phosphoric acid which may, for example, contain 2 to 20carbon atoms, especially 2 to 6 carbon atoms, such as alkenyl acids,e.g. vinyl sulphonic acid and vinyl phosphonic acid. Thus, the ester maybe methyl or ethyl vinyl sulphonate or phosphonate. The ester may bederived from an acid containing an ethylenically unsaturated carboxamide(e.g. acrylamido) group.

It is advantageous also to include in the copolymer, in addition tomonomers (i) and (ii) defined above, other monomers which impart hightemperature stability. for example 2-acrylamido-2-methylpropanesulphonic acid.

The ester is copolymerised with an ethylenically unsaturated polarmonomer to form the water-soluble copolymer. In the ethylenicallyunsaturated polar monomer, the unsaturated group is usually vinyl oralpha methyl vinyl, and may be derived from an unsaturated carboxylicacid (the acid being as further described above) e.g. as a primary,secondary or tertiary amide thereof, in which the amide is derived fromammonia, or a primary or secondary alkylamine, e.g. of 1 to 10 carbonatoms, which may optionally be substituted by at least one hydroxylgroup as in alkylol amides such as ethanolamides. Examples of suchcarboxylic derived polar monomers are acrvlamide, methacrylamide andacrylic ethanol amide. The polar monomer may also be a vinylheterocyclic compound e.g. with at least O, S or N atom in a ring with 3to 8 carbon atoms, such as one with at least one carbonyl group in thering, e.g. N-vinyl-pyrrolidone or -caprolactam, or a vinyl pyridine.

In general, the copolymer will contain 0.01 to 50%, e.g. 0.1 to 40% or 1to 30%, especially 5 to 15%, mol of structural units from said ester(s)and 99.99 to 50% e.g. 99.9 to 60% or 99 to 70% or 95 to 85%, mol ofstructural units from said polar monomer(s). The copolymer may be ablock or non-block copolymer, e.g. a regular or random copolymer or agraft copolymer, especially with ester units grafted onto polymericpolar, monomer, e.g. ester grafted on polyacrylamide.

The copolymer will usually be soluble in water to an extent of at least1 g/l e.g. 1 to 200 g/l such as at least 10 g/l in distilled water at15° C. especially in aqueous sodium chloride solution containing 32 g/lNaCl at 25° C. If desired, the copolymer may be mixed with a sufactant(e.g. in amount of 0.01 to 5% by wt of the solution) to help solubiliseit in the water or sodium chloride solution (e.g. brine).

The copolymer preferably has a weight average molecular weight of atleast 50,000 e.g. 50,000 to 20 million, such as 100,000 to 10 million,especially 100,000 to 500,000 or 1 to 10 million. The molecular weightmay be determined by conventional methods, e.g. gel permeationchromatography or intrinsic viscosity. The low molecular weightcopolymer may, for example, have a viscosity in an aqueous 3.6% byweight solution at 19° C. of 10 to 500 cps (measured at 60 rpm with aHaake viscometer). Preferably the copolymer is sheer thinnable, e.g.with the viscosity reducing by at least 10% on increasing the sheer rateby 10%.

The copolymer may be made by conventional methods for copolymerisingethylenically unsaturated monomers in solution, emulsion or suspension,(preferably aqueous), such as are described in Encyclopaedia of PolymerScience & Engineering, Ed. Mark, Bikales. Overberger and Menges, Publ.Wiley Interscience, New York. 2nd ed. vol 1, pp 181-211 and referencescited therein, especially L. J. Young in J. Brandrup and E. H. ImmergutEds, Polymer Handbook, J. Wiley, New York, 2nd Ed. 1975, Sec. II and 3rdEd. Sec. III, especially pp 155/6 and references cited therein and R. Z.Greenley, J. Macromol Science Chem. 14, 427, 445 (1980) and G. Saini etal., Makromol. Chem. 144, 235 (1971). Reference should be made to thesedocuments for further details. Free radical aqueous suspension andemulsion polymerisation is preferred.

The compositions of the invention comprise the copolymer and an organiccompound capable of cross-linking an acrylic acid polymer in aqueoussolution to form a gel. The cross-linking may be at 20-200° C.especially 40-150° C.

For downhole use, the composition of the invention contains water inwhich the copolymer and gelling agent may be dissolved. These aqueouscompositions are usually made up just before use by mixing the copolymerand the gelling agent in an aqueous medium. e.g. sea water, and theninjecting the aqueous composition into the formation. The composition ispreferably kept at below 50° C. e.g. below 30° C. before use. Theconcentration of copolymer in the aqueous composition is preferably from500 to 100,000 ppm, in particular 500 to 10,000 ppm for copolymers ofmolecular weight of at least 1 million, and from 10,000 to 100,000 ppmfor copolymers of molecular weight 50,000 to 1 million. Preferably, theconcentration of the gelling agent in the aqueous composition is from 10to 50,000 ppm, especially 10 to 1,000 ppm and 1,000 to 50,000 ppmrespectively, for the high and low molecular weight copolymers.

The aqueous compositions may be injected into a formation via aproducing well or via a secondary injection well (for use with a waterflood or squeeze technique), for example. The aqueous compositions mayalso contain other ingredients. e.g. antioxidants and/or oxygenscavengers. The injection may, if desired, be preceded by a precoolingtreatment, e.g. with cold water to stop premature cross-linking, butpreferably the injection process is performed without such apretreatment. The aqueous compositions may simply be injected into theformation but preferably they are forced into it by pumping. Theformation will usually be about 50° to 180° C. especially 60° to 100° C.or 100° to 150° C. and will generally be water-bearing rather thanoil-bearing. It may be of acidic rock, e.g. sandstone or neutral tobasic rock, e.g. limestone. with associated formation water of e.g. pH 3to 6.5 such as 4 to 6 or pH 6.5 to 8 respectively. The compositions ofthe invention are especially suitable for use with acidic and carbonaterocks, especially at about 60° C. to 150° C.

Compositions containing copolymers with carboxylic esters from tertiaryalkanols or arylmethanols, or from ether substituted alkanols orheterocyclic alcohols may be used for example with acidic rocks at80-120° C. and esters from other hydroxy compounds e.g. primary orsecondary alkanols at 40-80° C. The well may be shut in for 1-70 hours,for example, to allow the gelling to occur, and then production may berestarted.

The compositions of the invention have the benefit of a low tendency tocross-linking and gelling in the wellbore (i.e. reduced aggregatebuild-up) but rapid cross-linking at the high temperatures of theformation. They are, therefore, less susceptible to process handlingproblems, without the environmental and other problems associated withthe use of metal cross-linking agents.

The invention is illustrated in the accompanying figueres which showgraphical plots of viscosity against time for a series of compositions.The copolymers and cross-linking agent were placed in a Brookfield DV111Rheometer with external heating bath and the viscosity and timemeasured.

FIG. NO. Copolymer Cross-linker conc. Temperature 1 PA/PA t-butyl ester5000 ppm 90° C. 2 PA/MA 5000 ppm 90° C. 3 Partially hydrolysed PA 1% 90°C. 4 <0.1% hydrolysed PA 5000 ppm 90° C.

PA means polyacrylamide, MA means methyl acrylate, the cross-linker usedwas polyethyleneimine of average molecular weight 70,000, theconcentration is the concentration based on wt/wt. Copolymer 1 is acopolymer of polyacrylamide (95%) and tertiary butyl acrylate (5%).Copolymer 2 is a copolymer of polyacrylamide (95%) and methyl acrylate(5%). The partially hydrolysed PA corresponds to a copolymer of acrylicacid and acrylamide.

Copolymers 1 and 2 are according to the invention, but copolymers 3 and4 are not according to the invention and are given for comparison only.As can be seen, with the copolymer compositions of the invention, thereis gelling at the appropriate rate for use as a blocking agent in oilwell conformance.

In order that the invention may be more fully understood, the followingExamples are given by way of illustration only.

EXAMPLE 1

A 5 ft (1.52 m) stainless steel tube of internal diameter ¼ inch (0.635cm) was packed with quartz sand of average particle size 90 μm-150 μm.Sea water (3.2% total dissolved solids) was pumped into the tube at 40°C. until a constant differential pressure was obtained. The absolutepermeability of the sand pack was calculated by means of Darcy's Law tobe 1286 milli Darcies. The tube was then flooded with Forties Crude Oilcontaining 15% volume toluene followed by another flush of sea water toconstant differential pressure. The permeability of the sand pack atresidual oil saturation was calculated to be 436 milli Darcies.

A seawater solution containing 10% by weight of a 95:5 copolymer ofacrylamide and tertiary butyl acrylate and 4% polyethyleneimine waspumped into the tube (maintained at 40° C. by means of an oven) at aflow rate of 1.5 ml/hr and in such a way as to maintain a retention timeof ½ hour in the inlet tube at 40° C. prior to entry into the sand pack.The maximum pressure limit was set at 100 bar (10⁷ Pa) and this limitwas reached after 12 hours of flow after which virtually no further flowtook place.

In order to ascertain the depth of the gel block the tube was placedunder reverse flow while maintaining a differential pressure of 100 bar(10⁷ Pa). During this operation 6 inch (15.24 cm) sections of tube werecut from the outlet end (previously the inlet end) until the flow couldbe detected. The tube was found to be blocked to at least 4 ft 6 ins(1.37 m) which represented 95% of total volume pumped.

EXAMPLE 2

In an analogous fashion to Example 1, a 5 ft (1.52 m) tube was found tohave an absolute permeability of 1623 mD and a permeability of 463 mD atresidual oil saturation. A seawater solution containing 10% by weight ofthe copolymer and 2.5% by weight of polyethyleneimine was pumped at arate of 1-5 mls/hr and a temperature of 70° C. to give a total blockingtime of 4 hrs.

EXAMPLE 3

In an analogous fashion to Example 1, a 5 ft (1.52 m) tube was found tohave an absolute permeability of 198 mD and a permeability of 62 mD atresidual oil saturation (by employing sand of finer grain size). Aseawater solution containing 10% by weight of the copolymer and 0.4% byweight of polyethyleneimine was pumped at a rate of 3 mls/hr and atemperature of 110° C. to give a total blocking time of 5.5 hrs.

What is claimed is:
 1. A composition for use in a subterraneanformation, which composition comprises a water-soluble copolymercomprising (i) at least one non-acidic ethylenically unsaturated polarmonomer; and (ii) at least one coplymerisable ethylenically unsaturatedester; and at least one organic gelling agent, characterized in that theorganic gelling agent is a polyalkyleneimine, a poly-functionalaliphatic amine, an aralkylamine or a heteroaralkylamine.
 2. Acomposition according to claim 1, wherein the poly-functional aliphaticamine is a polyalkylenepolyamine.
 3. A composition according to claim 2,wherein the polyalkylenepolyamine is a polyethylene- orpolypropylene-polyamine.
 4. A composition according to claim 1, whereinthe polyakyleneimine is a polymerised ethyleneimine or propyleneimine.5. A composition according to claim 1, wherein the saidheteroaralkylamine contains at least one heteroatom.
 6. A compositionaccording to any of claims 1 to 5, wherein the copolymer comprises fromone to three polar monomers (i) and from 1 to 3 unsaturated esters (ii).7. A composition according to any of claims 1 to 5, wherein the ester isformed from a hydroxyl compound and an ethylenically unsaturatedcarboxylic acid selected from acrylic, methacrylic, crotonic andcinnamic acids.
 8. A composition according to claim 7, wherein thehydroxyl compound is an alcohol of formula ROH where R is a C1-30 alkyl,alkenyl, cycloalkyl, aryl or aralkyl group, is a hydroxyl, ether or thioether substituted hydrocarbyl group.
 9. A composition according to anyof claims 1 to 5, wherein the ethylenically unsaturated ester is analkyl or aralkyl acrylate in which the alkyl group is a C1 to 10 alkylgroup and the aralkyl group is a C1 to 5 alkyl substituted aryl group.10. A composition according to any of claims 1 to 5, wherein theethylenically unsaturated polar monomer is an amide of an unsaturatedcarboxylic acid.
 11. A composition according to claim 10, wherein theethylenically unsaturated polar monomer is acrylamide.
 12. A compositionaccording to any of claims 1 to 5, wherein the copolymer contains from5-15% mol of structural units from said ester and 95-85% mol ofstructural units from said polar monomer.
 13. A composition according toany of claims 1 to 5, wherein the copolymer is a regular or randomcopolymer or a graft copolymer.
 14. A composition according to claim 13,wherein the coploymer is a graft copolymer in which ester units aregrafted onto a polyacrylamide.
 15. A composition according to any ofclaims 1 to 5, wherein the copolymer is soluble in water to an extent ofat least 10 g/l.
 16. A composition according to any of claims 1 to 5,wherein the copolymer has a weight average molecular weight of at least50,000.
 17. A composition according to claim 16, wherein the copolymerhas a weight average molecular weight of 1-10 million.
 18. A compositionaccording to any of claims 1 to 5, wherein the copolymer and the gellingagent are in aqueous solution.
 19. A method of conformance control of asubterranean reservoir, which comprises: (a) injecting into a formationan aqueous solution of a composition as claimed in any of claims 1 to 5;(b) allowing the solution to flow through at least one permeable zone insaid formation; (c) allowing the composition to gel.